(a) Technical Field
The present invention relates to a liquid crystal display, and more particularly, to a liquid crystal display, an image signal modifying device, and an image signal modifying method.
(b) Description of the Related Art
A liquid crystal display (hereinafter referred to as an LCD) is one of the most widely used flat panel displays. LCD's generally include two display panels with a liquid crystal layer interposed therebetween. Electric field generating electrodes, such as pixel electrodes and a common electrode, are provided within the display panels. In the LCD, voltages are applied to the electric field generating electrodes to generate an electric field in the liquid crystal layer. Due to the generated electric field, liquid crystal molecules of the liquid crystal layer are aligned and polarization of incident light is controlled, thereby displaying images.
In general, the liquid crystal display includes a matrix of pixels each including a switching element realized with a thin film transistor (TFT). The LCD further includes a three terminal element and a display panel including display signal lines, i.e., a gate line and a data line. The thin film transistor functions as a switching element for transmitting a data voltage that is transmitted through the data line to a pixel or intercepting the pixel according to a gate signal that is transmitted through the gate line.
The liquid crystal capacitor has a pixel electrode and a common electrode as terminals and a liquid crystal layer between the electrodes functions as a dielectric material. A difference between a data voltage applied to the pixel electrode and a common voltage applied to the common electrode is referred to as a charged voltage of the liquid crystal capacitor, which is a pixel voltage. The arrangement of the liquid crystal molecules of the liquid crystal layer is changed according to the pixel voltage and in that way polarization of light passing through the liquid crystal layer is varied. The LCD additionally includes a polarizer for polarizing incident light. Accordingly, as the liquid crystal molecules change arrangement due to the applied electric field, the amount of polarized incident light that can transmit through the liquid crystal layer is affected. In this way, each pixel is able to provide a desired luminance, or gray level, in accordance with an image signal, by setting the pixel voltage.
However, since the response speed of the liquid crystal molecules is slow, it can take some time for the pixel voltage of the liquid crystal capacitor to reach a target voltage, which is a voltage used to acquire desired luminance. The length of time required for the liquid crystal capacitor to reach the pixel voltage is influenced by a difference between the voltage previously charged in the liquid crystal capacitor and the current target voltage. Accordingly, when the difference between the target voltage and the previous voltage is large, for example, when there has been no previous voltage applied, the liquid crystal capacitor may not reach the target voltage while the switching element is turned on.
Dynamic capacitance compensation (DCC) is a scheme in which the response speed of the liquid crystal is increased according to a driving method without changing the properties of the liquid crystal. DCC relies on the fact that the charging rate increases as the voltage at the liquid crystal capacitor becomes greater, and accordingly, the time required for the voltage in the liquid crystal capacitor to reach the target voltage is reduced by controlling the data voltage applied to the corresponding pixel to be greater than the target voltage. Where the common voltage is not zero, it is not necessarily the data voltage that is greater than the target voltage, but the difference between the data voltage and the common voltage that is greater than zero. However, for simplicity, it may be assumed that the common voltage is zero.
However, the drive frequency of the liquid crystal display is gradually increased, and as the drive rate of the liquid crystal display is increased, the time available to charge the liquid crystal capacitor is reduced. Therefore, the conventional DCC scheme may bring about degradation of image quality of the liquid crystal display, especially at high driving frequencies.